Alveolar macrophages recovered by bronchoalveolar lavage from individuals chronically exposed to inorganic dust were evaluated by light microscopy and by transmission and scanning electron microscopy. Alveolar macrophages from dust-exposed individuals contained a higher proportion of particles than did those of normal unexposed subjects. In addition, the surfaces of these macrophages showed features of activation, including increased numbers of rufflings, filopodia, pinocytotic vesicles, subplasmalemmal linear densities, and increased frequency of macrophage-macrophage and macrophage-lymphocyte interactions. These changes, which were associated with increased numbers of lysosomes, emphasize the important phagocytic role of macrophages in defending the lower respiratory tract.